: 
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XXXVIII. Observations on the Genus Cancer of Dr. Leach (Platycarcinus, Latr.,), with 
Descriptions of three new Species. By Tuomas Bext, Esq., F.R.S., L.S., G.S., & Z.S8. 
Communicated June 9, 1835. 
IN the course of the gradual distribution into various genera of a group of animals 
previously arranged under a single generic term, it is not always a matter devoid of 
difficulty to decide by which of the newly distinguished groups that original appellation 
should be retained ; and different rules have been laid down, and different principles 
resorted to, by various naturalists on this point, whilst others have been wholly careless 
on the subject. The consequence of this discrepancy has been the absence of all unity 
of design in the present heterogeneous nomenclature of the different divisions of the 
animal kingdom, according to the varying views adopted by the individuals by whom 
each portion has been separately studied and developed. 
It is undoubtedly desirable where a particular species can, with tolerable certainty, 
be recognised as having received a distinct appellation from any of the early masters of 
natural science, to retain that name for the genus to which the species belongs, and 
still to consider it as designating the smaller group in which it is included, whatever 
may be the changes and subdivisions made in the larger group to which it was origin- 
ally attached. This is still more imperative when the name has been so applied by any 
modern naturalist, whose character for learning and accuracy is such as to give weight 
to his opinion in matters of nomenclature. It appears to me that the name which I 
propose to retain for the genus which is the subject of this paper is thus strongly re- 
commended for our adoption, as being very probably the one by which the type of that 
genus was known to the older writers, and which has recently been applied by one of 
our most distinguished carcinologists, to the genus restricted by himself to the only 
species of it then known to him. The generic name Cancer was applied by Dr. Leach 
to the species Canc. Pagurus, with the full understanding that it constituted the type of 
a form distinct from all others of the family. I have therefore chosen this opportu- 
nity to claim for it the same distinction, upon the ground that the group was so desig- 
nated by my distinguished friend, before the term Platycarcinus was applied to it by 
Latreille in the French Museum, and consecrated by Dr. Milne Edwards in his recent 
admirable work on the natural history of Crustacea ; and also because, by applying any 
other term to this genus, we are obliged to restrict the word Cancer to a small and com- 
paratively unimportant group, not a single species of which was probably distinctly 
known to any naturalist of early times. 
When the characters of the present genus were first defined, the only known species 
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